1. Field of the Invention
The invention relates in general to a method of fabricating an implantation mask, and more particularly, to a method of fabricating an implantation mask using deep ultra-violet (UV) photo-resist.
2. Description of the Related Art
In FIG. 1, a method of mask implantation is shown. On the substrate 10, a thick photo-resist layer 12 is formed. The photo-resist layer 12 covers a part of the substrate 10. A part of the substrate 10 to be implanted with ion is exposed. Using the photo-resist layer 12 as an implantation mask, the exposed substrate 10 is implanted with ions. Since the material structure of the photo-resist layer 12 is unconfined, to avoid the implanted ions to penetrate therethrough, a very thick photo-resist layer is required. This is more obvious while implantation ion has a very high implantation energy. Moreover, it is difficult to coat a photo-resist layer with a thickness of about 2.5 .mu.m.
Under the condition of providing a plurality of photo-resist layers 22a, 22b, and 22c on a substrate 20 as shown in FIG. 2. Since the properties of these photo-resist layers 22a, 22b, and 22c are similar, so that a uniformity is obtained due to the mutual diffusion thereof. Therefore, while implanting the substrate with the photo-resist layers as masks, the implanted ions with a high implantation energy are very likely to penetrate through these photo-resist layers 22a, 22b, and 22c. In addition, the formation of multi-layers of photo-resist is very complex.
In FIG. 3, on a substrate 30, a sandwiched type photo-resist layer is formed. A photo-resist layer 32 is formed on the substrate 30. On the photo-resist layer 32, an ilayer 34. The photo-resist layer 32, the oxide layer 32, and the photo-resist layer 36 are stacked as a sandwiched type photo-resist layer. The photo-resist layer 36 is used to define the oxide layer 34, whereas the oxide layer 34 is a hard material to prevent ions to penetrate therethrough and implanted into the substrate 30 covered thereby. Though the formation of an oxide layer is advantageous to prevent ions to penetrate through, a high temperature is required during formation process, so that the compatibility between oxide and the photo-resist is poor. Therefore, a problem of integrity is caused during deposition of the photo-resist layer. Furthermore, the process for defining the oxide layer comprises a step of plasma etching which complicates the process of forming the sandwiched type photo-resist layer.
FIG. 4 shows the application of forming a well region of a twin-well structure within a substrate. On a substrate 40, a photo-resist layer 42 is formed and defined to cover a part of the substrate 40. The exposed part of the substrate 40 is doped by ion implantation with the photo-resist layer 42 as a mask. An annealing process is performed thereafter, so that the implanted ions are diffused and a well region 44 is formed. However, the diffusion range of the well region 40 formed by annealing is difficult to control. As the dimension of a device shrink, a well region with an outsize dimension affects the reliability of the devices seriously.
Another more advanced example of a well region formed by a conventional method is shown as FIG. 5. Considering the drawback of the well region formed by the above method, after the formation of a thick photo-resist layer 52 on a substrate 50, the exposed part of the substrate 50 is implanted with ion with higher energy. Therefore, the ions are implanted into a deeper region of the substrate 50. Consequently, an even diffusion is caused during annealing, an outsize range of a well region is avoided. However, as mentioned above, ions with a higher energy are very likely to penetrate through the photo-resist layer to dope a region which is predetermined as an undoped region or another well region doped with the opposite type of ions.